Metaphysical pluralism in philosophy is the multiplicity of metaphysical models of the structure and content of reality, both as it appears and as logic dictates that it might be, as is exhibited by the four related models in Plato's Republic and as developed in the contrast between phenomenalism and physicalism. Pluralism is in contrast to the concept of monism in metaphysics, while dualism is a limited form, a pluralism of exactly two models, structures, elements, or concepts. A distinction is made between the metaphysical identification of realms of reality and the more restricted sub-fields of ontological pluralism (that examines what exists in each of these realms) and epistemological pluralism (that deals with the methodology for establishing knowledge about these realms).